The electronic devices utilizing the phenomenon of superconduction find extensive utility in various applications as high-speed switches, high-sensitivity detectors, and high-sensitivity fluxmeters.
These superconducting devices are constructed with thin superconducting films. Since the thin superconducting films have considerably low critical superconducting temperatures (Tc) and, therefore, require use of liquefied helium as a cooling agent, they have encountered the problem of high cost, complexity of overall system, and incapability of size reduction.
Thus, studies have been promoted on thin oxide superconducting films possessing high critical superconducting temperatures. Thin oxide superconducting films discovered in recent years have critical superconducting temperatures exceeding 77.degree. K. and, therefore, are capable of being operated by the use of inexpensive liquefied nitrogen as a cooling agent.
For the production of thin oxide films of this class, the method which comprises superposing a given thin film by the spattering method or the vacuum evaporation method on a MgO single-crystal substrate or a SrTiO.sub.3 single-crystal substrate heated in advance to an elevated temperature has been used. It has been proposed in recent years for the purpose of enhancing the epitaxial growth of a thin oxide film to use as a substrate therefor an oxide insulator possessing a lattice-matching property relative to a given oxide superconductor and, at the same time, containing at least one of the component elements of the superconductor (Japanese Patent Application Disclosure SHO 63(1988)-236,794).
The conventional method for the production of a thin film by the use of a MgO single crystal or a SrTiO.sub.3 single crystal as a substrate does not easily produce an epitaxial film of high quality. This fact has posed itself a serious problem for the stabilization of critical superconducting temperature (Tc) and for the improvement and stabilization of the critical superconducting current (Jc).
To allow the growth of an excellent epitaxial film, the material for the substrate must fulfil the following requirements, for example:
(i) It should exhibit a highly satisfactory lattice-matching property to a thin crystal film. PA1 (ii) It should avoid deteriorating film quality due to mutual diffusion of the film and the substrate during the epitaxial growth of the film. PA1 (iii) It should possess a high melting point exceeding at least 1,000.degree. C. to withstand the heating at an elevated temperature. PA1 (iv) It should be procurable in the form of a single crystal possessing highly satisfactory crystallinity. PA1 (v) It should be an insulator of electricity.
Incidentally, numerous oxides such as those of the InBa.sub.2 Cu.sub.2 O.sub.7-.delta. (.delta.=0 to 1, Ln=Yb, Er, Y, Ho, or Gd) type, the Bi--Sr--Ca--Cu--O type, and the Tl--Ba--Ca--Cu--O type, for example, have been reported as high-temperature oxide superconductors. The lattice constants, a and b, of these oxide superconductors are invariably in the range of 3.76 to 3.92 .ANG..
Since they assume a face-centered configuration, the magnitudes .sqroot.2a and .sqroot.2b may well be regarded as representing the basic lattices. In this case, the lattice constants, a and b, are expressed as 5.32 to 5.54 .ANG..
In contrast, MgO which is a material now in widespread use for substrates has a lattice constant, a=4.203 .ANG., and a lattice mismatch ratio as high as to reach a range of 7 to 11%. Thus, it allows production of an epitaxial film of good quality only with difficulty.
SrTiO.sub.2 possesses a small lattice mismatch ratio in the range of 0.4 to 4% and exhibits an excellent lattice-matching property. The SrTiO.sub.3 single crystal, however, is produced at present solely by the Bernoulli method. The crystal obtained by this method exhibits very poor crystallinity and possesses an etch pit density exceeding 10.sup.4 pits/cm.sup.2. It allows production of an epitaxial film of high quality only with difficulty. Further, substrates of an appreciably large size are not procurable.